1. Field of the Invention
The present invention relates to a magnetic domain imaging system, i.e., an apparatus permitting one to observe magnetic domains in a magnetic specimen.
2. Description of Related Art
From the past, systems for obtaining transmission images of magnetic specimens using a transmission electron microscope have been known. In order to obtain an accurate image of a specimen, it is necessary to cause an electron beam transmitted through the specimen to be focused along the optical axis. FIG. 3 shows an example of configuration of the prior art instrument of the top entry type, i.e., a magnetic specimen is inserted from top to objective polepieces and held between them.
In FIG. 3, an electron beam 1 is deflected by two stages of deflection coils 2. An objective lens 8 magnifies a transmission image of the specimen. Indicated by 7 is an objective-lens coil. The objective lens has an upper polepiece 5 and a lower polepiece 6. A specimen 3 made of a magnetic material is placed in the upper polepiece where no magnetic field is present. Magnetic coils 4 apply a magnetic field to the specimen 3. The operation of the apparatus constructed in this way is described below.
The electron beam 1 emitted from an electron gun (not shown) is deflected by the two stages of deflection coils 2 and made to impinge on the specimen 3. At the same time, the magnetic coils 4 apply the magnetic field to the specimen 3. As a result, the magnetic domains in the specimen 3 are varied. The direction of magnetization is made different among individual domains. Under this condition, the beam transmitted through the specimen 3 passes through openings 9a and 9b formed in the polepieces. At this time, the objective lens field between the upper polepiece 5 and the lower polepiece 6 of the objective lens focuses the beam, forming a first transmission image. Then, it enters the imaging system (not shown) where the transmission image of the specimen is magnified in turn. Finally, the beam is focused onto a fluorescent screen or onto the sensitive surface of a CCD camera, thus permitting observation of the magnetic domains in the specimen.
FIG. 4 shows another example of configuration of the prior art instrument illustrated In JP-A-2007-80724 (paragraphs [0014]-[0022] and FIGS. 1 and 2). In FIG. 4, an electron beam 11 is deflected by a first deflector 12. There is also shown a second deflector 25. A second deflector coil (excitation coil) 25a is wound around the yoke of the second deflector 25.
A first principal deflection plane 14 is formed at the position of the first deflector 12. Indicated by 15 is a second principal deflection plane. A specimen 16 undergoes an inspection by an electron microscope, and is positioned at the front end of a specimen holder 17 of a magnetic field application mechanism having a gap 18 across which a magnetic field is applied. Also shown are an objective lens 19 and an objective-lens coil 27.
An objective lens gap 26 is placed in a stage following the specimen 16 and acts to serve a first focusing action immediately under the specimen. A coil 13 is wound around a front-end portion of the specimen holder 17 of the magnetic field application mechanism. The optical axis of the electron beam 11 is indicated by 30. The operation of the apparatus constructed in this way is as follows.
FIG. 5 is a schematic diagram illustrating the operation of the apparatus of the structure shown in FIG. 4. In FIG. 5, the specimen 16 undergoes the first focusing action of the objective lens 19 to form an objective lens image 20. The electron beam 11 impinges at an angle of incidence 21 on the specimen 16.
The electron beam 11 converged by a condenser lens (not shown) travels along the optical axis 30 and is slightly deflected by the first deflector 12 at the first principal deflection plane 14. On the other hand, the second deflector 25 is placed as close to the specimen 16 as possible. Therefore, the beam 11 is deflected at the second principal deflection plane 15 lying immediately above the specimen 16 and made to impinge at the on-axis center of the specimen 16.
In the specimen holder 17 of the magnetic field application mechanism, the specimen 16 is held in the magnetic field gap 18. Lines of magnetic force produced across the gap 18 apply a magnetic field to the specimen 16, and deflect the electron beam. A transmission electron image representing variations in the magnetic domains in the specimen due to the application of the magnetic field is focused as an objective lens image 20 by the lens action of the objective lens gap 26. Then, the objective lens image 20 is magnified by plural stages of focusing systems (not shown) until a desired magnification is produced. Finally, a high-magnification image is formed on an electron-beam dry plate, TV-like detector, or the like.
This kind of electron microscope permits observation of magnetic domains, the microscope having means to apply a magnetic field mounted within an objective lens, means to deflect and correct an electron beam mounted between the objective lens and an imaging lens, a means for selecting and applying an arbitrary phase of an alternating magnetic field, and a means for exciting an applied magnetic field using a synchronizing signal for image display means (see, for example, JP-A-8-96737 (paragraph [0007] and FIG. 1)).
The above-described prior art satisfies some key points in observing magnetic fields. That is, the magnetic field around the specimen is eliminated. The magnetic field applied to the specimen is controlled. However, there is the problem that it is difficult to make a correction for a greatly deflected electron beam in cases where the magnetic field is applied to the specimen.